Sounds interesting... not exactly sure what you mean by "carrying slightly larger pure white ones on top of them". Are there two distinctly different kinds of insects here?
There are a large number of "bugs" that inhabit soil, and most of them are quite harmless or even beneficial. You'll have to do some research in order to find out exactly what you have going on. It helps if you can observe the bugs and get a feel for their appearance\behavior. For example: the mites that I have in my soil also become more mobile upon watering. They are very small (about 1-2 mm), orange-tan colored and they have an exoskeleton (micro-arthropods). They mostly tend to stick around just beneath the soil surface; but when populations are high they will swarm the edges of the containers and immediately surrounding areas. They also seem to like to crawl up my left arm, if present. I would have to say that there might potentially be hundreds of them, per container.
Your bugs might not be arthropods although mites (orbatids, other very small soil mites) and springtails (they jump around like fleas) are common soil arthropods; but your might be soft bodied. Some pest insects also spend all/part of their life cycle in the soil and damage roots
